The present invention relates generally to electrical water heaters. More particularly, the invention relates to methods and apparatus for determining whether a heating element of a water heater is surrounded by water.
A storage-type water heater typically comprises a permanently enclosed water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating material in at least a portion of the annular space for providing thermal insulation to the water tank. The water tank has various appurtenances such as inlet, outlet and drain fittings. Additionally, the water heater is provided with a water heating and temperature control system. The water heating and temperature control system includes an electrical-resistance-heating element. The heating element extends through a fitting in the wall of the water tank such that the heating element is inside the tank. The heating element is connected to an electrical power source outside the water tank.
One of the problems associated with water heaters having electrical-resistance-heating elements is when the heating element xe2x80x9cdry firesxe2x80x9d. The term xe2x80x9cdry firexe2x80x9d refers to the heating of a heating element that is not submerged in water. Usually, a dry fire will destroy or burn-out the heating element in less than a minute. Therefore, it would be beneficial to determine whether a heating element is submerged in water before utilizing that element to heat the water.
Accordingly, the invention provides a water heater having a water heater controller operable to detect when a heating element is not submerged in water. The controller of the invention determines whether a heating element is surrounded by water by applying a voltage to the element and calculates an initial and subsequent resistance for the element. As the element heats up, the resistance of the element increases. If the element is in water, the element reaches equilibrium temperature very quickly. If the element is xe2x80x9cdryxe2x80x9d, it gets continuously hotter and reaches a high temperature in a short period of time. Thus, to determine if the element is surrounded by water, the controller measures a beginning and ending resistance, and compares the measured resistance values. If a difference between the two resistance values is small, then the controller determines that the element is xe2x80x9cwetxe2x80x9d (i.e., surrounded by water). If a difference between the two resistance values is large, the controller determines that the element is xe2x80x9cdryxe2x80x9d (i.e., not surrounded by water). If the controller determines an element is xe2x80x9cdryxe2x80x9d, then the controller will not use that element to heat the water.
The invention further provides a method of determining whether water is surrounding a heating element of a water heater. The method includes the acts of applying a signal to the heating element, measuring a first resistance of the heating element, measuring a second resistance of the heating element after measuring the first resistance, and determining whether the heating element is surrounded by water by comparing the first heating resistance with the second resistance.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.